This invention relates to variable frequency drives and, more particularly, to a fast stopping method for an induction motor operating from a variable frequency drive.
A motor drive system, in one known form, comprises an AC source supplying three-phase AC power to a variable frequency drive (VFD). The VFD includes an AC/DC converter connected by a DC link or bus to a DC/AC converter. The DC/AC converter may comprise a pulse width modulated inverter using insulated GATE bipolar transistors (IGBTs).
Speed and torque control of induction motors using VFDs has become universally accepted in the industry. Speed control includes stopping as well as reducing speed of a rotating load. Many applications require rapid stopping of a rotating motor. There are two (2) common stopping methods used in adjustable speed drives (ASDs). These are coasting and decelerated stop. In the coasting method, the control signals for turning on and off the inverter IGBTs are turned off and no voltage is provided to the motor. The motor coasts to a stop. The time taken for the motor to come to rest depends upon the inertia of the rotor-load combination. If the inertia is not large, then the rotor takes a long time to come to rest. In the decelerated stopping method, the motor is commanded to operate at a reduced frequency, and thus speed. The commanded operating frequency (speed) is gradually reduced to bring the motor to rest. Due to the inertia of the rotor-load combination, the speed cannot be reduced instantaneously. Generally, the user can select a predetermined rate of deceleration, which can be adjusted depending on the application. Since the commanded speed is lower than the actual rotating speed of the rotor-load, the motor starts behaving like an induction generator. In other words, the motor enters negative slip operation. Slip is defined as the ratio of the difference between the commanded speed and actual speed to the commanded speed. The commanded speed is also known as synchronous speed xcfx89s. Mathematically, slip is defined as follows:                     s        =                                            ω              s                        -                          ω              r                                            ω            s                                              (        1        )            
In equation (1), xcfx89r is the rotor speed or the actual rotating speed of the rotor-load combination. Note that slip, s, is a dimensionless quantity.
The mechanical energy in the rotor-load inertia is converted into electrical energy by the induction generator action. This energy is typically absorbed by the DC bus capacitors present in all ASDs. The voltage across the DC bus rises. Eventually, the excess energy is dissipated in the bleeding resistors present across the DC bus capacitors. In many cases, the energy in the moment of inertia is very large and it takes a long time for the energy to bleed off into bleed resistors. This delays the time it takes to stop a rotating load with large inertia. If the user selects a low deceleration time to bring the rotor-load combination to a rapid stop, then the rate of increase in voltage across the DC bus capacitors could be higher than the time constant of the bleed resistor and DC bus capacitor combination. This could result in an over-voltage trip condition at which time the VFD reverts to a coasting method to stop. Nuisance trips similar to those described above are highly undesirable and result in loss of production due to machine downtime. In order to circumvent this situation, many users employ external IGBT-resistor combinations. Such a unit is known as a xe2x80x9cbrake unitxe2x80x9d. The IGBT is turned on when the voltage across the DC bus exceeds a predetermined value. The excess energy is thus dissipated in the external resistors. The size and cost of the external IGBT-resistor combination is an added burden to the end user. The method of employing external resistors to achieve relatively faster stopping time is known as dynamic braking and the resistors are known as DB resistors.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention, there is provided a fast stopping method for induction motors operating from variable frequency drives.
It is an object of the invention that the braking method does not cause an over voltage trip in the VFD.
It is another object of the invention that the braking method does not require the use of additional brake-units or regenerative units.
It is still another object of the invention that the method does not require use of a tachometer or encoder feedback.
It is still a further object of the invention that the method has a stopping time that is better or comparable to the method using additional brake units or regenerative units.
In accordance with the invention, the above objects are achieved in a fast stopping method known as large slip braking (LSB). The proposed method relies upon maintaining large slip between the commanded speed and the actual rotor speed to achieve fast stopping times.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.